One of the main objectives of a video conferencing system is to provide an effective vehicle for interaction between remote persons. The ability to see and interact with different conference participants is essential to the conduct of any meeting. Visual presence is therefore one of the most important criteria a video conferencing system must satisfy. It is also desirable to see who is looking at whom during the meeting.
All video conference systems utilize, at each site, at least one monitor and at least one camera for displaying the remote conferee(s) and for obtaining an image of the local conferee(s). A persistent problem which arises from current configurations of these components in a video conferencing system is the so-called "parallax effect", which prevents the appearance of eye contact between the speaker and those remote viewers to whom he or she is speaking. Essentially, this effect results from the placement of the camera relative to the monitor and viewer.
The parallax effect is illustrated in FIG. 1. As seen in FIG. 1, each of conference participants A and B views a respective display 12, 14, having a windows for presenting the image of the other participant. Associated with each display is a video camera for capturing the image of the corresponding viewer. Each camera may be placed above, below, or to either lateral side of the associated display. In the illustrative prior art system shown in FIG. 1, cameras 16 and 18, respectively, are placed directly above displays 12 and 14. The angle .theta. represents the angle between camera 16 and display 12 relative to viewer A. Because participant A is looking directly at the displayed image of participant B, and therefore not at camera 16, the displayed image of A appears to be looking down. The parallax effect has a similar impact on the displayed image of participant B.
As will be readily ascertained by those skilled in the art, the parallax effect can be minimized by placing the camera and monitor as close together as possible. In fact, previous efforts to eliminate the parallax effect have relied upon positioning the camera directly behind a specially constructed display screen. In U.S. Pat. No. 4,054,908 issued to Poirier et al on Oct. 18, 1977, for example, the image of the observer is reflected to the camera lens by an oblique reflecting plate. The display screen is viewed by the observer through a window in the reflecting plate. The presence of the window in the apertured plate, however, is somewhat distracting to the viewer. It has also been proposed to utilize an electronic crystal screen which alternates between opaque and transparent states at a frequency slow enough to permit an image of the viewer to reach the camera while in the transparent state and to present a displayed image while in the opaque, reflecting state. Disadvantageously, images displayed on such a flashing screen are characterized by unacceptably low brightness. Moreover, the screen structure and associated driving circuitry are costly to fabricate.
So-called virtual space video conferencing systems have been proposed in an effort to provide full visual presence and convey the spatial relationship between provide three or more conference participants. In this type of system, the number of monitor and camera pairs at each site corresponds to the total number of remote conference participants, and the relative positions of the pairs relative to each participant are fixed at each site. An illustrative system of this type is depicted in FIG. 2. Each viewing subject (A,B,C, and D) sits in a swivel chair in a corresponding room (1,2,3, or 4) with three TV monitors representing the other three "virtually present" participants. Thus, for example, the viewing subject (A) sits in a swivel chair in Room 1 with three TV monitors representing the three "virtually present" participants (B,C, and D). The arrangement of monitor/camera pairs in each room duplicates the positions of each participant as if they were all seated at the same square table. The signals from these cameras pass through video switching units and are directed to corresponding destination monitors by a dynamic allocation network.
The virtual space system of FIG. 2 provides three or more participants with the ability to see who is looking at whom during a conference. Eye contact between the participants, however, has heretofore been obtained by resort to the apertured or semi-reflecting screen devices discussed above. Moreover, the large number of video channels needed to provide the desired spatial distribution between the conference participants translates into a large expenditure of transmission bandwidth.